The purpose of this invention is a process for manufacturing boron nitride fibres, and particularly continuous boron nitride fibres with good mechanical properties that can be used to make ceramic composite materials such as BN/BN composites, thermostructural parts and antenna radomes.
More precisely, it concerns obtaining boron nitride fibres from a polymer precursor that is shaped by spinning to form polymer fibres that are then ceramised to transform them into boron nitride fibres.
There are many processes for making boron nitride, as described by R. T. PAINE et al in chem. Rev., 90. 1990, pages 73-91 [1]. In particular, the methods described in this document include processes using precursor polymers formed from organic boron compounds such as borazines.
One way of obtaining this type of precursor polymers was described by C. K. Narula et al in Chem. Mater, 2, 1990, pages 384-389 [2]. It consists of making trichloroborazine or 2-(dimethylamino)-4, 6-dichloroborazine react with hexamethyldisilazane in solution in dichloromethane at ambient temperature.
If 2-(dimethylamino)-4,6-dichloroborazine is used, polymerisation at two points is encouraged due to the presence of the NMe2 group. It is noted that the term xe2x80x9cpolymerisationxe2x80x9d is the British spelling for the term xe2x80x9cpolymerization,xe2x80x9d and that both these terms mean the same, namely, the process of forming polymer.
Another method of obtaining precursor polymers described in EP-A-0 342 673 [3] consists of making a B-tris (inferior amino alkyl) borazine react with an alkylamine such as laurylamine, either thermally in mass or in solution.
Other precursor polymers can also be obtained by thermal polycondensation of trifunctional aminoborazines with formula [xe2x80x94B(NR1R2)xe2x80x94NR3xe2x80x94]3 in which R1, R2 and R3 are identical or different and represent hydrogen, an alkyl radical or an aryl radical as described in FR-A-2 695 645 [4].
The polymers described above are suitable for obtaining powder or other forms of boron nitride, but it is more difficult to prepare more complex forms, and particularly fibres from this type of polymers.
Frequently, the precursor polymer necessary for shaping the fibres is drawn badly due to its statistical reticulated structure which causes only a slight elongation, making control of the fibre section very random. Later on in the process, this causes breakages of fibres or weak points, which results in very weak final mechanical properties.
As indicated by T. Wideman et al in Chem. Mater., 10, 1998, pp. 412-421 [5], research has been continued to find other precursor polymers that are more suitable for obtaining boron nitride fibres. This document describes that a spinnable precursor polymer in the molten state may be obtained by modifying polyborazylene by reaction with a dialkylamine or with hexamethyldisilazane.
The purpose of this invention is a process for manufacturing boron nitride fibres using other precursors to obtain fibres with satisfactory mechanical properties.
According to the invention, this result is achieved using a borazine in which the three boron atoms are substituted by amino groups, at least one of which is different, as the precursor monomer.
According to the invention, the process for manufacturing boron nitride fibres by spinning of a precursor polymer and ceramisation of the polymer fibres obtained by spinning, is characterised in that the precursor polymer is obtained by thermal polymerisation of a borazine of formula (I): 
in which R1, R3, R4 and R5 that may be identical or different, represent an alkyl, cycloalkyl or aryl group, and
R2 represents a hydrogen atom or an alkyl, cycloalkyl or aryl group.
In this process, the choice of a borazine with formula (I) to form the precursor polymer leads to an approximately linear polymer. The fact that the borazine used is an asymmetric borazine concerning amino groups present on its boron atoms, encourages links between monomer patterns along two lines so that a reticulated polymer is not obtained, inducing a proportion of direct intercyclic links in the polymer.
In the borazine used in the invention, the R1 to R5 groups may represent alkyl, cycloalkyl or aryl groups. Alkyl and cycloalkyl groups may have 1 to 30 carbon atoms, and preferably from 1 to 10 and even better 1 to 4 atoms of carbon. For ceramisation, it is preferable to limit the number of carbon atoms in substitutes to obtain a better conversion rate to boron nitride.
Aryl groups that could be used in the invention may be groups comprising one or several phenyl radicals, and phenyl and benzyl groups are used in preference.
According to one preferred embodiment of the invention, R2 in formula (I) represents a hydrogen atom. The result is then a dysfunctional precursor comprising two NHR amino groups where R is an alkyl, cycloalkyl or aryl group, and a tertiary amino group. This arrangement is favourable for obtaining a polymer with better spinning performances.
Also preferably, the remaining R1, R3, R4, R5 groups are methyl groups since they facilitate good ceramic efficiency.
Also according to a first embodiment of the invention, borazine complies with formula (I) in which R2 represents a hydrogen atom and R1, R3, R4, and R5 represent the methyl group. Therefore, this is [2,4-bis(monomethylamino)-6-dimethylamino]borazine.
According to a second embodiment of the invention, borazine complies with formula (I) where R1 to R5 represent the methyl group corresponding to [2,4-bis(dimethylamino)-6-monomethylamino]borazine.
These borazines may be synthesised by the process described by B. Toury et al in Main Group Met. Chem. 22, 1999, pp. 231-234 [6]. In this document, it was shown that polymerisation of borazines of the same type at moderate temperatures (140 to 145xc2x0 C.) leads to polymers with direct Bxe2x80x94N links between two borazine radicals. On the other hand, linearity of the polymer was not observed.
This work should have encouraged an expert in the subject to decide not to use this type of borazine to obtain precursor polymers with a better behaviour in spinning, since the presence of direct links should have been negative for spinning since the polymer was less flexible.
On the contrary, it is observed with this invention that this type of structure is very attractive since it is actually very close to the structure of the ceramic. Furthermore, this arrangement limits aggregation of cycles during polymerisation, which finally results in a non-rigid and easier to spin pseudo-linear polymer. Furthermore, it is easy to move the amino-labile groups remaining on the polymer chain during ceramisation.
According to the invention, thermal polymerisation of borazine with formula (I) is carried out preferably at a final temperature exceeding 140xc2x0 C., for example from 160 to 190xc2x0 C. It is possible to operate under argon in an autogenous atmosphere, in other words to retain an atmosphere of amines that are compounds released during thermolysis, above the polymer. Polymerisation can also be done under an inert gas flow (rare gas or nitrogen) or under a vacuum, by adapting temperatures and durations. Usually, since the initial borazines put into the reactor may contain a certain quantity (up to 20% by weight) of a synthesis solvent such as toluene, it is preferable firstly to dry the monomer under a primary vacuum before carrying out the polymerisation step. This drying may be done at a temperature from 30 to 80xc2x0 C., to eliminate the synthesis solvent.
During the polymerisation step, the eliminated volatile products can be analysed continuously, either by pHmetry or by gaseous chromatography to control the polymerisation operation. These volatile products can also be trapped at low temperature and then analysed by the usual spectroscopic techniques.
Heating programs and durations and the atmospheres used depend on the borazine used in formula (I).
After the polymerisation step, a polymer is obtained with a vitreous transition temperature of less than 100xc2x0 C., so that spinning is possible at temperatures less than 200xc2x0 C.
The polymer can be spun using conventional techniques, using nozzles including one hole only or several holes. The fibre leaving the nozzle may be wound onto graphite reels. Preferably, spinning is done in an inert atmosphere, for example under a nitrogen atmosphere. The polymer fibres are ceramised after spinning. When the reels are not treated immediately, they can be kept in an inert chamber or under a vacuum.
For ceramisation of the fibres, the temperatures, heating rates, durations and the atmosphere used are chosen as the function of the precursor polymer used and the result to be obtained.
Preferably, ceramisation is done in two steps.
The first preceramisation step consists of heating the fibres, for example up to a temperature of less than or equal to 1000xc2x0 C., and preferably from 400 to 600xc2x0 C. in an NH3 atmosphere.
The second ceramisation step itself is carried out by increasing the temperature of the preceramised fibres to a higher level of at least 1400xc2x0 C., for example from 1400xc2x0 C. to 2200xc2x0 C.
This step is done under a nitrogen and/or a rare gas atmosphere in one or several operations, and possibly with intermediate cooling at ambient temperature.
For example, this step may be carried out under a nitrogen atmosphere at a temperature from 1600 to 1800xc2x0 C. and under a rare gas atmosphere beyond this temperature.
Another purpose of this invention is continuous boron nitride fibres obtained using the process described above, characterised in that they have an average breaking stress ("sgr"R) of 1000 to 2000 MPa and the Young""s Modulus E is between 80 and 250 GPa.
Other characteristics and advantages of the invention will be better seen after reading the following examples, obviously given for illustrative purposes and in no way restrictive.